This disclosure relates to non-volatile storage.
Semiconductor memory has become increasingly popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrically Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories. With flash memory, also a type of EEPROM, the contents of the whole memory array, or of a portion of the memory, can be erased in one step, in contrast to traditional EEPROM.
For storage elements, some semiconductor memory devices use transistors that have a control gate (“CG”) and a floating gate (“FG”), with the floating gate being used to store one or more bits of information. The floating gate is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between the source and drain regions. The control gate is provided over and insulated from the floating gate (“FG”). The threshold voltage (VTH) of the transistor thus formed is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
For some devices, programming is achieved by applying one or more programming pulses to the control gate, which may cause electrons from the channel to be injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory cell is raised so that the memory cell is in a programmed state. Reading the state of a storage element may be achieved by applying one or more read reference voltages to the control gate and sensing a conduction current of the storage element. A relatively strong conduction current indicates that the storage element has a threshold voltage that is below the read reference voltage.
Semiconductor memory devices also have transistors that are used for purposes other than storage elements. Some transistors are used as selection devices to select which groups of storage elements are programmed or read. For example, in an architecture commonly referred to as the NAND architecture, there are “NAND strings” that include a number of storage elements in series with two or more “select gate transistors.” Applying an appropriate voltage to the control gates of the select gate transistors can be used to select a group of storage elements. Oftentimes, one storage element of the selected group is selected for programming or reading. For example, one storage element on a NAND string might be selected for programming or reading. Note that typically many NAND strings are selected together, such that many storage elements may be programmed or read together.
It may be desirable for the control gates to have low resistance. Some memory devices may utilize metal for at least a portion of the control gates of the storage elements, which may help to decrease resistance. However, fabricating memory devices with metal control gates presents challenges. One challenge is that metal can be difficult to etch. Therefore, it can be desirable to have a semiconductor fabrication process that does not require etching metal to form control gates. Some semiconductor fabrication processes intentionally form a silicide when forming the control gates, which may help to reduce resistance. However, forming a silicide can also have drawbacks.